Subject: H7) How does the ocean respond to a hurricane and how does
this feedback to the storm itself?

Contributed by Joe Cione (HRD)

The ocean's primary direct response to a hurricane is cooling of the
sea surface temperature (SST). How does this occur? When the strong
winds of a hurricane move over the ocean they churn-up much cooler
water from below. The net result is that the SST of the ocean after
storm passage can be lowered by several degrees Celsius (up to 10°
Fahrenheit).

Figure 1 shows SSTs ranging between 25-27°C (77-81°F)
several days after the passage of Hurricane Georges in 1998. As
Figure 1 illustrates, Georges' post-storm 'cold wake' along
and to the right of the superimposed track is 3-5°C (6-9°F)
cooler than the undisturbed SST to the west and south (i.e. red/orange
regions are ~30&deg'C [86&deg'F]). The magnitude and distribution of
the cooling pattern shown in this illustration is fairly typical for
a post-storm SST analysis.

One important caveat to realize however is that most of the 3-5°C
(6-9°F) ocean cooling shown in Figure 1
occurs well after the storm has moved away from the region (in this
case several days after Georges made landfall). The amount of ocean
cooling that occurs directly beneath the hurricane within the high
wind region of the storm is a much more important question scientists
would like to have answered. Why? Hurricanes get their energy from
the warm ocean water beneath them. However, in order to get a more
accurate estimate of just how much energy is being transferred from
the sea to the storm, scientists need to know ocean temperature
conditions directly beneath the hurricane. Unfortunately, with
150kph+ (100mph+) winds, 20m+
(60ft+) seas and heavy cloud cover being the norm in this region of
the storm, direct (or even indirect) measurement of SST conditions
within the storm's "inner core" environment are very rare.

Thankfully in this case "very rare" does not mean "once in a
lifetime". Recently, scientists at the Hurricane Research Division
were able to get a better idea of how much SST cooling occurs directly
under a hurricane by looking at many storms over a 28 year period. By
combining these rare events, HRD scientists put together a "composite
average" of ocean cooling directly under the storm.

Figure 2 illustrates that, on average, cooling
patterns are a lot less than the post storm 3-5°C (6-9°F)
cold wake estimates shown in Figure 1. In most cases, the ocean
temperature under a hurricane will range somewhere between 0.2 and
1.2°C (0.4 and 2.2°F) cooler that the surrounding ocean
environment. Exactly how much depends on many factors including
ocean structure beneath the storm (i.e. location), storm speed,
time of year and to a lesser extent, storm intensity (Cione and
Uhlhorn 2003).

While the estimates in Figure 2 represent
a dramatic improvement when
it comes to more accurately representing actual SST cooling patterns
experienced under a hurricane, even small errors in inner core SST can
result in significant miscalculations when it comes to accurately
assessing how much energy is transferred from the warm ocean
environment directly to the hurricane. With all other factors being
equal, being "off" by a mere 0.5°C (1°F) can be the difference
between a storm that rapidly intensifies to one that falls apart!
With that much at stake, scientists at HRD and other government and
academic institutions are working to improve our ability to accurately
estimate, observe and predict "under-the-storm" upper ocean conditions.
These efforts include statistical studies, modeling efforts and
enhanced observational capabilities designed to help scientists better
assess upper ocean thermal conditions under the storm. With such
improvements, it is believed that future forecasts of tropical cyclone
intensity change will be significantly improved.